CourseWiki/Materials and Manufacturing Processes/Metals

Metal Stock Forms and Purchasing

Recognize stock shapes, distinguish size conventions, and specify enough information to receive the metal a design requires.

Stock is the material shape you buy before making the part

Stock is material supplied in a usable starting form. Choose a shape that leaves enough material for the finished part and any cutting or machining allowance. Buying the smallest possible shape can fail if saw cuts, surface cleanup or distortion consume the allowance.

Cross-sections of round, flat, square and hex bar, round and square tube, angle, channel and I-beam. Solid bar has no central opening; tube has a wall around an opening. — Cross-sections show shape only. Dimensions and wall thickness still belong on the order.

Sheet, plate, bar and structural shapes use different size descriptions

What to specify for common forms
Form	Description	Size information
Sheet and plate	Flat products; plate is the thicker category under the applicable product convention.	Thickness, width and length. There is no single sheet/plate boundary for every metal and specification.
Bar	Solid round, flat, square or hexagonal cross-section.	Diameter, width and thickness, square side, or hex size across flats, plus length.
Structural shapes	Angle, channel and I-shaped sections place material where the structure needs it.	Section designation and dimensions, thicknesses and length under the relevant product system.

A gauge number is a sizing convention, not a universal thickness. Specify actual thickness and units when ambiguity would affect the part. Check the applicable material's gauge table if a drawing uses gauge.

Tube dimensions and nominal pipe sizes are not interchangeable

Tube is commonly ordered by outside dimensions and wall thickness: outside diameter for round tube, or outside width and height for rectangular tube. State the length too.

Pipe commonly uses a nominal pipe size and a wall schedule. A nominal size is a label; it need not equal the measured outside or inside diameter. Use the applicable pipe dimensions table before calculating a fit or flow area. Structural tubing is not automatically suitable for pressure service.

Measure the right boundary. For round hollow stock, inside diameter equals outside diameter minus twice the wall thickness. The wall occurs on both sides of the opening.

Hot-rolled and cold-finished describe processing, not alloy identity

Hot-rolled steel is shaped at high temperature and commonly arrives with mill scale and comparatively broad dimensional tolerances. Cold rolling or other cold-finishing operations can improve surface finish and dimensional control. Cold-finished bar may be drawn rather than rolled.

Cold work can increase strength and leave residual stresses. Removing material can release those stresses and move the part. Cold-finished does not automatically mean precision ground, and neither term guarantees a particular tolerance without a product specification.

Why an attractive surface can still be the wrong purchase

A shiny bar can have the wrong alloy, condition, straightness or size allowance. Conversely, inexpensive hot-rolled stock can be a sound choice when later machining removes the scale and establishes the final dimensions. Compare total processing cost, not appearance alone.

A purchase description must preserve alloy, condition, form and size; Checklist

A purchase description must preserve alloy, condition, form and size

An illustrative request is: 6061-T6 aluminum round bar, 25 mm nominal diameter × 500 mm cut length, quantity 2. Before purchasing, add the governing product specification, diameter and cut-length tolerances, finish and certification requirements that the design needs. This example is not a complete engineering specification.

Identify the alloy and required delivery condition from the design.
State the stock form and every necessary dimension with units.
Allow for saw kerf (material lost in the cut), cleanup and final machining.
State tolerances, quantity and any required material certificate.
Compare availability, cut charges, shipping and minimum orders as well as material price.
Confirm that the delivered label and documents match the order before using the stock.

Drops or remnants are leftover pieces that may reduce cost. Their identification and usable dimensions still need checking.

Estimate mass from cross-sectional area, length and density; Practice

Estimate mass from cross-sectional area, length and density

For uniform stock, mass = cross-sectional area × length × density. For a solid round bar, area = πd²/4. For round tube, area = π(D² − d²)/4, where D is outside diameter and d is inside diameter. Subtract the opening before multiplying by length.

Use the density for the actual alloy from a material data sheet and keep units consistent. A result in kilograms is mass; weight as a force also requires gravitational acceleration. See Geometry, Mass, and Force and Dimensional Analysis.

Practice bar mass with a supplied density Practice tube volume

Appears in

DET 130 DET Metals and Heat Treatment